Floating body memory cell having gates favoring different conductivity type regions

ABSTRACT

A method for fabricating floating body memory cells (FBCs), and the resultant FBCs where gates favoring different conductivity type regions are used is described. In one embodiment, a p type back gate with a thicker insulation is used with a thinner insulated n type front gate. Processing, which compensates for misalignment, which alloys the different oxide and gate materials to he fabricated is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/017,432, filed Feb. 5, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/644,167, filed Mar. 6, 2015, now U.S. Pat. No.9,275,999, issued Mar. 1, 2016, which is a continuation of US. patentapplication Ser. No. 14/028,043, filed Sep. 16, 2013, now U.S. Pat. No.8,980,707, issued Mar. 17, 2015, which is a divisional of U.S. patentapplication Ser. No. 13/534,985, filed Jun. 27, 2012, now U.S. Pat. No.8,569,812; issued Oct. 29, 2013, which is a divisional of U.S. patentapplication Ser. No. 11/644,715, filed Dec. 22, 2006, now U.S. Pat. No.8,217,435, issued Jul. 10, 2012, the entire contents of which are herebyincorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to the field of dynamic, random-access memories(DRAMs), and devices with double gates, particularly those usingtransistors with floating bodies, also known as floating body cells(FBCs).

PRIOR ART AND RELATED ART

Most common DRAM cells store charge on a capacitor and use a singletransistor for accessing the capacitor. More recently a cell has beenproposed which stores charge in a floating body of a transistor. A backgate is biased to retain charge in the floating body.

In one proposal, an oxide layer is formed on a silicon substrate and asilicon layer for the active devices is formed on the oxide layer (SOIsubstrate). The floating bodies are defined from the silicon layer andthe substrate is used as a back or biased gate. One problem with thisarrangement is the relatively high voltage required on the back gatebecause of the thick oxide. For this structure and others, when FBCs arescaled to state-of-the-art gate lengths, it is necessary to use eitherhigh voltage back gate bias or thinner back gate oxide to retain theextra holes in the body. The holes collected at the back gate interfacedepends on the back gate/flat-band potential difference and the gateoxide thickness. As the oxide is thinned, the gate leakage becomes high,causing the tunneling of electrons, which has the effect of erasing thestored charge.

Several structures have been proposed to reduce the relatively high biaspotential discussed above, including use of a double gate floating bodyand silicon pillars. These structures are difficult to fabricate. Thisand other related technology is described at C. Kuo, IEDM, December2002, following M. Chan Electron Device Letters, January 1994; C. IEDM.December 2002, “A Hypothetical Construction of the Double Gate FloatingBody Cell;” T. Ohsawa, et al., IEEE Journal of Solid-State Circuits,Vol. 37, No. 11, November 2002; and David M. Fried, et al., “ImprovedIndependent Gate N type FinFET Fabrication and Characterization,” IEEEElectron Device Letters, Vol. 24, No. 9, September 2003; Highly ScalableFBC with 25 nm BOX Structure for Embedded DRAM Applications, T Shino,IDEM 2004, pgs 265-268; T. Shino, IEDM 2004, “Fully-Depleted FBC((Floating Body Cell) with enlarged signal Window and excellent LogicProcess Compatibility; ” T. Tanaka, IEDM 2004, “Scalability Study on aCapacitorless IT-DRAM: From Single-gate PD-SOI to Double-gate FinDRAM;U.S. patent application Ser. 2005/0224878; and Independently Controlled,Double Gate Nanowire Memory Cell with Self-Aligned Contacts,” U.S.patent application Ser. No. 11/321,147, filed Dec. 28, 2005.

Another floating body memory formed on a bulk substrate is described inSymposium on VLSI Technology Digest of Technical Papers, page 38, 2005by R. Ranica, et al. The floating p well, as described, is isolated fromneighboring devices by a shallow trench isolation region and underlyingn well. Drain disturbance occurs when devices on the same column areread or written. A parasitic bi-polar transistor between the source,drain and body; and between the source, body and n well, can causecharge loss under disturb conditions. As will be seen in one embodimentof the present invention, this problem is addressed. Other problemsassociated with the high voltage bias are also addressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a prior art floating body cell (FBC) and itsconnection to the peripheral circuits in a memory.

FIG. 2 is a perspective view of a prior art FBC fabricated on asilicon-on-insulator (SOI) substrate.

FIG. 3 is a perspective view of a FBC in accordance with one embodimentof the present invention, fabricated on a SOI substrate.

FIG. 4 is an energy diagram showing, the accumulated hole density fordifferent back gate biases for an n+ work function gate and a p+ workfunction gate.

FIG. 5 is a diagram showing the Wentzel-Kramers-Brillouin (WKB)approximation used to calculate the transmission probability ofelectrons from the gate to floating body of FIG. 6.

FIG. 6 is a diagram showing the potential across the back gate oxideversus the transmission probability of electrons.

FIG. 7A is a cross-sectional, elevation view of a substrate where finsfor FBCs separated by isolation regions are defined in all n well. FIGS.7-15 are generally through a section line corresponding to line 7-7 ofFIG. 3, although unlike the SOI substrate of FIG. 3, a bulk substrate isused.

FIG. 7B is a cross-sectional, elevation view of a different section ofthe substrate of FIG. 7A, were isolation regions are formed in a p welland in an n well; this section of the substrate is used for thefabrication the logic CMOS transistors.

FIG. 8A illustrates the structure of FIG. 7A, after the isolationregions are etched.

FIG. 8B illustrates the structure of FIG. 8A, after the isolationregions are etched.

FIG. 9A illustrates the structure of FIG. 8A, after a dielectric layeris formed.

FIG. 9B illustrates the structure of FIG. 8B, after a dielectric layeris formed.

FIG. 10A illustrates the structure of FIG. 9A, after the formation andplanarization of a SLAM layer.

FIG. 10B illustrates the structure of FIG. 9B, after the formation andplanarization of a SLAM layer.

FIG. 11A illustrates the structure of FIG. 10A, after a masking step.

FIG. 11B illustrates the structure of FIG. 10B after removal of the SLAMlayer.

FIG. 12A illustrates the structure of FIG. 11A, after etching stepswhich selectively remove the SLAM layer and underlying oxide layer, andwhich remove masking members and remaining SLAM.

FIG. 12B illustrates the structure of FIG. 11B, after removal of theoxide layer.

FIG. 13A illustrates the structure of FIG. 12A, after the formation ofan additional oxide layer.

FIG. 13B illustrates the structure of FIG. 12A, after formation of anoxide layer.

FIG. 14A illustrates the structure of FIG. 13A, after another maskingstep and SLAM etching step and the removal of p metal from exposedregions

FIG. 14B illustrates the structure of FIG. 13B, after removal of the pmetal from the p well region.

FIG. 15A illustrates the structure of FIG. 14A, after deposition of an ametal layer, polysilicon layer and planarization.

FIG. 15B illustrates the structure of FIG. 14B, after deposition of an ametal layer, polysilicon layer, and planarization.

FIG. 16A is a cross-sectional, elevation view of the structure shown inFIG. 15A, however, taken spaced-apart from the gate regions (generallythrough a section line corresponding to line 16-16 of FIG. 3), and afteranother masking step, SLAM etching step, and during tip ionimplantation.

FIG. 16B illustrates the structure of FIG. 15B, taken spaced-apart fromthe gate regions during tip ion implantation,

FIG. 17 is a cross-sectional, elevation view of the FBCs following theformation of suicide, generally through a section line corresponding tosection line 7-7 of FIG. 3.

FIG. 18 is a plan view of another embodiment of a memory employing FBCs,where different oxide thickness and a bottom gate with a different workfunction than a top gate are used.

FIG. 19 is a cross-sectional, elevation view through two cells in thememory, taken through section line 19-19 of FIG. 18.

FIG. 20 is a cross-sectional, elevation view of two cells in the memoryof FIG. 18, taken through section line 20-20 of FIG. 18.

FIG. 21 is a cross-sectional, elevation view showing processing used tofabricate the FBCs of FIGS. 19 and 20, as seen through the section line19-19.

FIG. 22 is a cross-sectional, elevation view showing processing used tofabricate the FBCs of FIGS. 19 and 20, as seen through the section line20-20 of FIG. 18.

FIG. 23 illustrates the structure of FIG. 21, after the formation ofoxide regions.

FIG. 24 illustrates the structure of FIG. 23, after the formation of thebottom gate, which corresponds to the back gate in a FBC.

DETAILED DESCRIPTION

In the following description, a memory and method for fabricating thememory is described. Numerous specific details are set forth, such asspecific conductivity types, to provide a thorough understanding of thepresent invention. It will be apparent to one skilled in the art, thatthe present invention may be practiced without these specific details.In other instances, well known processing steps and circuits have notbeen described in detail, in order not to unnecessarily obscure thepresent invention.

Floating Body Cell Operation and Prior Art Devices

A single memory cell is shown in schematic form in FIG. 1. A portion ofa semiconductor line, body or fin 120, formed on an oxide layer (such asBOX 250 of FIG. 2), and etched from, for example, a monocrystallinesilicon layer is illustrated. The body 120 includes a pair ofspaced-apart, doped regions 110 and 130, disposed at opposite ends ofthe body thereby defining a channel region 100. In one embodiment, thechannel region is a p type region, and the source region 130 and drainregion 110 are more heavily doped with an n type dopant. The channelregion may be doped to two different doping levels adjacent to itsopposite sides.

A pair of gates identified as a front gate 140 and back gate 150 areformed about the body 120. The gates 140 and 150 are insulated from thechannel region 100 of the silicon body 120 by the oxide layers or high kdielectric layers 160 and 170, respectively. In FIG. 1 the gates areshown on opposite sides of the body to simplify the figure. A moreaccurate depiction of the cell is shown in perspective view in FIG. 2.The cell is typically formed in an array of cells in a memory.

The memory cell of FIG. 1 is a four-terminal device, coupled to theperipheral of circuits of the memory. For the n channel embodimentillustrated, the source region is coupled to ground, and the back gate150 is coupled to a source of bias (a constant potential), for example,−1 volt. The drain terminal 110 is connected to a bit line 230 in thememory. The front gate 140 is connected to a word line 240 in thememory, to allow selection of the cell. The cell, as will be described,is a dynamic, random access memory cell, and as such, the data storedrequires periodic refreshing.

Assume first, that the cell of FIG. 1 is not storing charge, and thatthe cell is selected by the application of a positive potential to aword line which is coupled to the gate 140. Assume further, that abinary one is to be stored in (written into) the cell as represented bythe storage of charge. (A binary 0 is represented by the absence ofcharge.) An amplifier 190 provides a positive potential to the bit line230 causing conduction in the inversion channel 210 of the channel 100of the body 120, as typically occurs in a field-effect transistor. Asthis occurs, hole generated from the impact ionization for an n channelembodiment (resulting generally from impact ionization) drift towardsthe gate 150, under the influence of the bias applied to this gate.These holes remain in the storage 200 of the body region 120 after thepotential is removed from the word line 240 and the potential is removedfrom the bit line 230. Other charging mechanisms may be used to writedata into a cell. For example, gate-induced drain leakage (GIDL) alsocreates electron/hole pairs at a different set of biases (VFG<O, Vd>0,VBG<0).

Assume that it is necessary to determine whether the cell is storing abinary 1 or binary 0. The cell is selected by the application of apositive potential to the word line 230. The threshold voltage of thecell shifts, depending on whether holes are stored in the region 200.The cell has a lower threshold voltage, that is, it conducts morereadily, when there is charge stored in the region 200. This shift inthreshold voltage is sensed by the sense amplifier 180 and provides areading of whether the cell is storing a binary 1 or binary 0. Thereading is provided to an 110 output line, or to refresh circuitry torefresh the state of the cell.

The threshold voltage of the cell maybe determined by comparing, theread current to a reference current in a cross-coupled sense amplifier.The reference current may be established by averaging over a pair ofreference cells with one cell in state “1” and the other in state “0”.

One characteristic for a dynamic memory cell is its retention time. Thisis the time between refresh cycles needed to restore the stored chargerepresenting a binary state. Ideally, the retention time should be aslong as possible to reduce the overhead associated with refreshing thecells and to provide longer periods during which the cells may beaccessed. Ideally, the retention time should be increased withoutnegatively impacting other cell characteristics such as read voltage,cell size, etc. As will be seen below, improved retention time isobtained without an increase to the cell size or its bias voltage. Thisis achieved by asymmetrical gate structures, as will be described.

In one prior art FBC, the cell is fabricated on a BOX 250 of FIG. 2; BOX250 is formed on a silicon substrate not illustrated. Active devices forthe memory are fabricated in, for instance, a monocrystalline siliconlayer, disposed on the BOX 250. This SOI substrate is well-known in thesemiconductor industry. By way of example, it is fabricated by bonding asilicon layer onto a substrate, and then, planarizing the silicon layerso that it is relatively thin. This relatively thin, low body effectlayer, is used for active devices. Other techniques are known forforming the SOI substrate including, for instance, the implantation ofoxygen into a silicon substrate to form a buried oxide layer. In theprior art device of FIG. 2, the gates 140 and 150 are illustrated alongwith the fins 120 and source region 130.

Embodiment of FBC With Asymetrical Gate Structure and it's Benefits

Referring now to FIG. 3, in one embodiment of the FBC, a fin 264 isfabricated on a buried oxide 260. A source region 263 of the fin 264 isillustrated. A front gate 261 and back gate 262 are shown separated by asilicon nitride member 265. Unlike the structure of FIG. 2, in FIG. 3the gate structures 261 and 262 are different. The work function of themetals for each of the gates is different and/or the gate oxidethicknesses are different. For an n channel embodiment, the gate 261 canbe an n+ doped polysilicon gate or a metal having a work functionfavoring an n channel device, while the gate 262 can be a p+ dopedpolysilicon gate or a metal having a work function favoring a p channeldevice. Below these gates are referred to as an n+ gate and p+ gate,respectively. When fabricated from metal, a high-k dielectric istypically used.

A gate dielectric having a high dielectric constant (k), such as a metaloxide dielectric, are for instance, HfO₂ or ZrO₂ or other high kdielectrics, such as PZT or BST. (Referred to below as gate oxides.) Thegate dielectric may be formed by any well-known technique such as atomiclayer deposition (ALD) or chemical vapor deposition (CVD). Alternately,the gate dielectric may be a grown dielectric. For instance, the gatedielectric, may be a silicon dioxide film grown with a wet or dryoxidation process.

For an n channel embodiment, the p+ gate has a thicker oxide to preventthe transmission of charge and hence, improve retention. Moreover, nosource/drain tip implant region is formed on the back gate side of thefin.

The metal gate is formed over the gate oxide. In one embodiment, a gatematerial comprises a metal film such as tungsten, tantalum, titaniumand/or nitrides and alloys thereof. For the n channel device, a workfunction in the range of 3.9 to 4.6 eV may be used. For the p channeldevice, a work function of 4.6 to 5.2 eV may be used. Accordingly, forsubstrates with both n channel and p channel transistors, two separatemetal deposition processes may need to be used. The remainder of thegate may be of another metal or polysilicon, as occurs in one embodimentbelow.

A comparison of the hole accumulations for the devices of FIGS. 2 and 3is shown in FIG. 4. The arrow 150 points to a line representing the boleaccumulation associated with the gate 150 of FIG. 2 as a function ofgate voltage. The backgate 150 is assumed to be an n+ gate. Similarly,the arrow 262 points to the line associated with the hole accumulationfor the gate 262 of FIG. 3. The backgate 262 is assumed to be a p+ gate.Assume a voltage of −1.5 volts on the gate 150 of FIG. 2, the same holedensity can be achieved with a voltage of only −0.4 volts for the gate262 of FIG. 3. The p+ gate attracts substantially more holes for a givenvoltage than its counterpart n+ gate of FIG. 2.

The WKB approximation of FIG. 5 of the tunneling barrier from the bodyto the gate was used to develop the transmission probability versusvoltage graph of FIG. 6. In FIG. 6, the probability of charge transferthat effectively erases the stored charge is illustrated. FIG. 6illustrates three approximations: one for a p+ gate, another for an n+gate, and the intermediate case with a mid-gap gate. As can be seen, thetransmission probability is about four orders of magnitude higher forthe n+ gate which would be the case for the embodiment of FIG. 2 whencompared to using the p+ gate of FIG. 3. Even the mid-gap gate providesan improvement of two orders of magnitude. The transmission probabilitybears directly on the retention time. With a lowered electrontransmission probability, the retention time in the FBC is improved asin the case of the p+ back gate in an n channel FBC.

One challenge in realizing the structure of FIG. 3, particularly wherethe fins are formed in an array at the critical dimension of a process,is masking for the fabrication of two different gate oxides and/or gatematerials. Because perfect alignment is seldom achievable in a maskingprocess, some mechanism is generally used to compensate formisalignments. As will be described below, compensation is provided formisalignments, thereby permitting the fabrication of the device of FIG.2, at the smallest geometries associated with a given process. Moreover,as will be described below, the FBCs are fabricated on the same bulksubstrate as the logic devices.

Fabrication of FBC With Assymetrical Gate Structures

The described processing below focuses on the fabrication of FBCs in amemory array. While the array is fabricated on one section of anintegrated circuit, the peripheral circuits for the memory or otherlogic circuits such as would be used for a processor are fabricated onother sections. Moreover, while the description below is directed to thefabrication of the cell on a bulk substrate, other substrates may beused such as SOI substrate shown in FIG. 3.

FIG. 7A illustrates a section of the p type substrate 300 upon which theFBCs and logic circuits are fabricated. The substrate for thisembodiment is an ordinary monocrystalline p type silicon (bulk)substrate. (Note the term “floating” body is used for bodies formed onbulk; even though such bodies are not intuitively floating as they arewith an SOI substrate.) The memory devices are fabricated in an n well310 formed below the upper region of the substrate which remains p type.FIG. 7B illustrates other portions of the substrate. P wells 312 areformed where a channel transistors are to be fabricated. N wells 314 areformed where p channel transistors are to be fabricated. It will beappreciated that the n wells 310, p wells 312, and n wells 314 may bedispersed on the substrate so that logic transistors of the desiredconductivity type can be placed where needed.

The substrate 300 has a pad oxide 320, initially grown on the substrate,as is typically done. Then, a silicon nitride layer is deposited on thesubstrate, masked and etched to form hard masking members 325, shown inFIG. 7A, and corresponding members not shown in FIG. 7B. These membersare used to allow definition of fins both in the memory array section,as well as in the logic section. Ordinary trench processing is used toform the trenches 315 between the nitride members 325, again both in thememory section and logic section of the substrate. A planarization stepis used to provide the flat surfaces shown in. FIGS. 7A and 7B.Following this, the silicon nitride members are removed, only in thelogic section. This is the point in the processing shown in FIGS. 7A and7B.

Next, as shown in FIGS. 8A and 8B, a plasma (dry) etching step is usedto etch back the trench oxide regions 315 of FIGS. 7A and 7B. This is atimed etch, leaving some of the trench isolation 315, as shown in FIGS.8A and 8B. When this occurs, the fins 350 of FIG. 8A are revealed. Thesefins are used for the FBCs. Similarly, fins 330 in the p well 312 andfins 340 in the n well 314 are similarly revealed. Note that thisetching step also removed the oxide 320 in the logic section, whereasthe oxide 320 remains in the memory section because of the protectionfrom by the silicon nitride members 325. The fins 330 are used for the nchannel logic, tri-gate transistors, whereas the fins 340 are used forthe p channel logic tri-gate transistors.

Referring now to FIGS. 9A and 9B, a first gate dielectric layer 326 isformed with a blanket deposition over the entire substrate. A grownoxide may instead be used. In one embodiment, this is a deposited layerof silicon dioxide or other oxides. As will be seen, this dielectriclayer is subsequently removed except where the back gates of the FBCsare formed, and it is this layer which provides the extra thickness ofinsulation for the back gate.

A sacrificial light absorbing material (SLAM) layer 360 is now formedover the entire substrate using, for instance, a spin-on process. Othersacrificial layers may be used instead of a SLAM. The SLAM 360 is shownin both FIGS. 10A and 10B after it has been planarized.

As illustrated in FIG. 11A, masking members 361 are formed from aphotoresist layer over adjacent pairs of the fins 350 in the arraysection of the substrate. The masking members 360 leave exposed theregion between every other fin in the memory array section. No maskingmembers are formed at this time in the logic section of the array. Next,the exposed SLAM layer is etched with an ordinary wet etchant, leavingthe structure shown in FIGS. 11A and 11B.

It should be noted in FIG. 11A, that it will be difficult to preciselyalign the masking members 361 with the edges of the fin structures. Moretypically, the mask will not be in perfect registry with the underlyingfins. The dotted lines 362 in FIG. 11A show a typical mask alignment,with the mask shifted to the left with respect to the underlyingstructure. Because a wet etchant is used, the SLAM nonetheless beremoved in the region shown by the arrows 363. This tolerance for maskmisalignment allows, as will be seen, a practical process for providingdifferent gate structures on opposite sides of each FBC.

Now, the photoresist members 361 are removed and a wet etching stepfollows to remove all exposed oxide, both in the array section and thelogic section of the substrate. Note, if the oxide used is SiO₂ it isremoved prior to the removal of the members 361. If the oxide is ahigh-k material, it may be removed after the members 361 are removed.Then, the remaining SLAM is removed resulting in the structure shown inFIGS. 12A and 12B. In FIG. 12A, it can be seen that the oxide 326remains between alternate pairs of the fins 350, shown as regions 366,and no oxide remains between the intermediate regions 365 shown in FIG.12A. Thus, looking at the parallel, spaced-apart fins of FIG. 12A, thesurfaces facing each other from two adjacent fins have a dielectric(within regions 366), whereas the next two facing surfaces with regions365 do not have a dielectric. As will be described, the regions 366 areused for the back gates for the FBCs. The FBCs are arranged such thatone cell has its back gate on the right of the fin, and the next cellhas its back gate on the left of the fin. No oxide remains on the fins330 and 340 in the logic section, as shown in FIG. 12B.

A gate oxide 367 is next formed over the entire substrate, this oxidewill be the gate oxide for both the p and n channel transistors in thelogic section, and the gate oxide for the front gates of the FBCs (seeFIGS. 13A and 13B). Again, this oxide may be any insulator such as ahigh-k material previously discussed. For the back gate (regions 366) ofthe FBCs, there are now two oxide layers 326 and 367, thus providing thethicker oxide needed to prevent the transfer of charge as shown inconjunction with FIGS. 5 and 6.

A blanket deposition of a gate metal layer with a work functionappropriate for a p type device or a polysilicon p-doped polysilicongate layer is now formed over the entire substrate, including both thememory section and the logic section. Layer 375, if a metal is used, hasa work function appropriate for a p channel device (e.g., 4.6 to 5.2 eV)to obtain the benefits described in connection with FIG. 4. Then,another SLAM layer is deposited and the substrate is planarized. Themasking step shown in FIG. 11A is again repeated. However, this time,masking members are also formed over the n well 314 so as to protect thep metal for the p channel transistors. A wet etch is again used toremove the exposed SLAM and the p metal which is not protected by theSLAM. To allow tighter design rule, the SLAM may be first etched with adry etch, followed by a wet etch to reduce the space required betweenthe pmos and nmos if it were all wet etched. The resultant structure isshown in FIGS. 14A and 14B. In FIG. 14A, the resultant SLAM members 370in the memory section protect the regions 366. As can be seen beneaththe SLAM 370, there is a p metal layer 375. Similarly, the SLAM maskingmember 370, covering the n well structures of the logic section, protectthe p metal 375 which will subsequently be used for the Rates of the pchannel transistors.

The metal gate material 375 is shown extending continuously over twoadjacent fins 340 in FIG. 14B. Later in the processing, a gate is formedover contiguous fins in p well 312. Most often in the logic section ofthe substrate the gates are formed so that they extend only over asingle tin so as to form individual transistors. In some cases, a singlegate drives two or more transistors, as shown. It will be appreciatedthat the spacing of the fins can be varied or other processing used toform individual tri-gate transistors in the logic section.

Following formation and selective etching of layer 375, what remains ofthe SLAM 370 is removed. An n metal gate material is now deposited overthe substrate. This metal is deposited over the p metal as well as overthe gate oxide for the n channel devices. The work function for the pmetal remains unaffected by the overlaying of the n metal for the pchannel devices and for the back gate of the FBCs.

Next, there is a blanket deposition of a polysilicon layer 380, followedby planarization, resulting, in the structure shown in FIGS. 15A and15B. After Planarization, the gate can be patterned to as desired gatelength in the direction perpendicular to the cross section shown in thefigures. Note in the regions 366, the back gates of the FBCs have twolayers of oxide (326 and 367) and two metal layers, first the p metal375 and the overlying a metal 376. In the regions 365, the front gate ofthe FBCs, there is only a single layer of oxide 367 and a single layerof the n metal 376. Each back gate serves two adjacent cells, andsimilarly, each front gate serves two adjacent cells.

Referring again to FIG. 3, the processing described for FIGS. 7-15involve the formation of the gate structures in the array section, andcorresponding gate structures in the logic section. Hence, the views inthese figures are through the gate regions. FIGS. 16A and 16B arecross-sectional, elevation views, however, taken through the region ofthe fin, spaced apart from the gates as generally shown by section line16-16 of FIG. 3. Note that at the stage of the processing shown in FIG.16A, the oxide layer 320 and silicon nitride layer 325 is still on thefins, and as will be seen, this helps facilitate a tip implant.

Now, the SLAM and masking; step of FIGS. 10A and 11B are repeated toform the SLAM members 390 shown in FIG. 16A. Also, an ordinaryphotoresist layer 391 is masked and etched over the n well logic sectionof the substrate to protect the sites of p channel devices. Two angledion implantation steps are used to form the n type tip source and drainregions, as shown in FIGS. 16A and 16B. Because of the members 390, onlyone side of the fins 350 are implanted, this side corresponding to theregion adjacent the front gates of the FBCs. These tips implantedregions in the fins 350 alternate between the right and left sides ofthe fins because of the back-to-back arrangement of the cells asdescribed above.

Ordinary processing is next used to fabricate tri-gate and dual-gatedevices in the logic and memory sections, respectively, including tipimplant for the p channel devices in the logic section, halo implants(if used), and formation of spacers to allow the doping of the mainsource and drain region for both the n channel and p channel devices.

Finally, as shown in FIG. 17, a suicide or salicide is formed on thepolysilicon to complete the front and back gates.

Several alternative processing, steps and orders of steps, may be usedto provide the above-described structure. For instance, while as shownin FIG. 13A, the thick oxide 326 was formed followed by the thinneroxide 367, these processes may be reversed. The thin oxide 367 can befirst formed, and the SLAM layer used to protect it, while a thickeroxide, is formed for the back gates. Similarly, while in FIGS. 14A and14B, the p metal gate was first formed and then protected by the SLAMlayer where needed, the n metal gate could first be formed and protectedby the SLAM layer for the n channel devices followed by the formation ofthe p metal. Other alternative processing steps and orders may be usedwith the above-described process.

Embodiment With Bottom Back Gate and Top Transistor

FIG. 18 is a plan view of an alternate embodiment where the memory arrayincludes a bottom gate which performs the functions of the back gate,for the previously described embodiments. The bottom gate 415 of FIG. 18surrounds the fins as will be seen, and is biased to retain the holeswithin the FBCs. A top gate functions as the word line for the FBCs; thebit lines are connected to the drains in a direction orthogonal to theword lines. Individual cells need not be isolated from each other,however, diffusion isolation may be used with a small impact on layoutarea using a cut mask. Even with the isolation between transistors, cellareas can be realized smaller than those associated with independentdouble-gates, due to the elimination of contacts to the back gate andfront gate for each cell or cell pair. Moreover, only two metal layersare needed to connect the array, in part, because there is no need forseparate gate contacts per cell or cell pairs.

Two completed cells, formed in an n well 400, viewed through the sectionlines 19-19 of FIG. 18, are illustrated in FIG. 19. Fins 410, doped witha p type dopant, and etched or grown from a bulk monocrystallinesubstrate are shown. The bottom gate 415, as mentioned, surrounds thefins and provides the bias for retaining the charge within the fins 415.The transistors for the FBCs are formed in the upper part of the fins415 and include the doped a type source and drain regions 420, as willbe described. FIG. 20 is an orthogonal view to that of FIG. 19, andagain shows the fins 410. The bottom gate is insulated from the well 400by the oxide 418, and from the top gate 429 by the oxide 430 asillustrated in both FIGS. 19 and 20.

Referring to FIGS. 21 and 22, the memory, as mentioned, for theillustrated embodiment, is realized on a bulk substrate, however, it mayalso be realized on an SOI substrate. An a well 400 is first implantedinto a p type bulk wafer in the regions where the memory array is to befabricated. Then, a thin layer of pad oxide 462 is deposited or grownacross the wafer, followed by an isolation nitride deposition, as istypically used for a shallow trench isolation process. The trenchisolations in the memory array section, can be first patterned bymasking off the sections of the wafer used for logic devices. As analternative, the isolation in the logic area can be processed at thesame time as the memory section, followed by the removal of the bottomgate from the logic section while the bottom gate in the memory sectionis protected.

After removal of the trench isolation, there are a plurality of fins410, as shown in FIGS. 21 and 22, formed in the n well 400, and cappedwith an oxide 462 and the silicon nitride hard masking members 461. Now,an insulator such as silicon dioxide or a polymer layer is deposited,planarized and etched back to leave a layer of insulation at the bottomof the isolation trenches, as shown as insulation 418 in FIG. 23. Thisinsulation is used to avoid the turning on of parasitic transistorsbetween neighboring devices, as shown by the line 465 in FIG. 23. (Thisproblem was mentioned in the prior art section.) The insulation 418 maynot be necessary depending upon the thickness of a bottom gate oxide andthe doping level of the n well 400. The bottom gate oxide is formed inthe bottom of the insulation trenches and on the sides of the fins 410.

Next, the gate oxide for the bottom gate is grown, for instance, in adry atmosphere, on the surfaces 419 of FIG. 23. This oxide, for thereasons described in conjunction with FIGS. 5 and 6, is relatively thickto prevent the loss of charge between the bottom gate and the storageregion of the fins 410. A polysilicon layer is now deposited to form thebottom gates 415. This is a blanket deposition of polysilicon which isplanarized, and etched back to provide the polysilicon bottom gate 415of FIG. 24. A vertical implantation step can be used to dope thepolysilicon. While the polysilicon can be doped with an n type dopant(for an n channel FBC) for the reasons shown in FIGS. 3, a p type dopantis preferred. Before and/or after the formation of the bottom gate,angled implants can be used to adjust the doping level in the p wells ofthe fins 410. Following this, the isolation trenches can be filled,planarized and etched back to provide the insulation 430 shown in FIGS.19 and 20.

Known processing can next be used to fabricate tri-gate transistors orplanar transistors in the upper regions of the fins 410. This can bedone, as an example, using a replacement gate process where a tipimplantation with an n type dopant is used followed by the formation ofthe spacers 425 of FIG. 19, prior to the doping of the main source anddrain regions 420. The source and drain regions 420 are not deep enoughto short to the n well 400. Some overlap between the source and drainregion and the bottom gate is permissible, as the bottom gate is biasedto accumulate charge in the floating body. By biasing the bottom gate sothat charge accumulates, the gate cuts off the parasitic bi-polartransistor that would otherwise exist between the source and drain,p-body and n well. This improves the charge retention in a disturbedcondition. While in the embodiment illustrated, the transistor is atri-gate transistor, a planar transistor can be formed in the uppersurface of the fins 410.

In either event, a more traditional silicon dioxide polysilicon gate maybe used or a high-k insulator and metal gate favoring an n type workfunction may be used. Note that since the top gate is formed separatelyfrom the bottom gate, the gate dielectric thicknesses between the twocan be different, allowing a thicker bottom gate insulator to improveretention time.

Thus, several embodiments of an FEW have been described where differentgate insulation thickness and gate material within each cell is used.

What is claimed is:
 1. A semiconductor structure, comprising: asemiconductor substrate comprising a P well region having a firstsemiconductor fin protruding therefrom and an N well region having asecond semiconductor fin protruding therefrom, the first semiconductorfin spaced apart from the second semiconductor fin, wherein the P wellregion is directly adjacent to the N well region in the semiconductorsubstrate; a trench isolation layer disposed on the semiconductorsubstrate between the first and second semiconductor fins, wherein thefirst and second semiconductor fins extend above the trench isolationlayer; a gate dielectric layer disposed on the first and secondsemiconductor fins and on the trench isolation layer, wherein the gatedielectric layer is continuous between the first and secondsemiconductor fins; an n type metal gate layer disposed over the gatedielectric layer over the first semiconductor fin but not over thesecond semiconductor fin, wherein the n type metal gate layer is furtherdisposed on a portion of but not all of the trench isolation layer; andp type metal gate layer disposed over the gate dielectric layer over thesecond semiconductor fin, wherein the p type metal gate layer is furtherdisposed over the trench isolation layer and over the n type metal gatelayer.
 2. The semiconductor structure of claim 1, wherein the p typemetal gate layer comprises titanium nitride.
 3. The semiconductorstructure of claim 1, wherein the n type metal gate layer comprisestitanium.
 4. The semiconductor structure of claim 1, wherein the p typemetal gate layer has a work function in the range of 4.6 to 5.2 eV. 5.The semiconductor structure of claim 1, wherein the n type metal gatelayer has a work function in the range of 3.9 to 4.6 eV.
 6. Thesemiconductor structure of claim 1, wherein the gate dielectric layercomprises HfO₂.
 7. The semiconductor structure of claim 1, wherein the ptype metal gate layer comprises titanium nitride, the n type metal gatelayer comprises titanium, and the gate dielecrtric layer comprises HfO₂.8. The semiconductor structure of claim 1, wherein the semiconductorsubstrate is a bulk silicon semiconductor substrate.
 9. A method offabricating a semiconductor structure, the method comprising: forming aP well region and an N well region in a semiconductor substrate, whereinthe P well region is directly adjacent to the N well region; forming afirst semiconductor fin in the P well region and forming a secondsemiconductor fin in the N well region, the first semiconductor finspaced apart from the second semiconductor fin; forming a trenchisolation layer on the semiconductor substrate between the first andsecond semiconductor fins, wherein the first and second semiconductorfins extend above the trench isolation layer; forming a gate dielectriclayer on the first and second semiconductor fins and on the trenchisolation layer, wherein the gate dielectric layer is continuous betweenthe first and second semiconductor fins; forming an n type metal layerover the gate dielectric layer; patterning the n type metal layer toform an n type metal gate layer over the gate dielectric layer over thefirst semiconductor fin but not over the second semiconductor fin,wherein the n type metal gate layer is further formed on a portion ofbut not all of the trench isolation layers, and forming a p type metalgate layer over the gate dielectric layer over the second semiconductorfin, wherein the p type metal gate layer is further formed over thetrench isolation layer and over the n type metal gate layer.
 10. Themethod of claim 9, wherein the p type metal gate layer comprisestitanium nitride.
 11. The method of claim 9, wherein the n type metalgate layer comprises titanium.
 12. The method of claim 9, wherein the ptype metal gate layer has a work function in the range of 4.6 to 5.2 eV.13. The method of claim 9, wherein the it type metal gate layer has awork function in the range of 3.9 to 4.6 eV.
 14. The method of claim 9,wherein the gate dielectric layer comprises HfO₂.
 15. The method ofclaim 9, wherein the p type metal gate layer comprises titanium nitride,the n type metal gate layer comprises titanium, and the gate dielectriclayer comprises HfO₂.
 16. The method of claim 9, wherein thesemiconductor substrate is a bulk silicon semiconductor substrate.